Meshed telemetry system using frequency hopping for intermittent transmission

ABSTRACT

A plurality of cooperative telemetry systems is disclosed in which each telemetry system comprises a plurality of remote telemetry transmitters to intermittently transmit short duration messages indicative of the status of sensors associated with the remote telemetry transmitters and at least one telemetry collection unit including a radio transceiver. Each telemetry collection unit forms a node in a wireless network in which telemetry collection units cooperate with each other in order to increase reliability and lower the average cost of each telemetry system. Such network can communicate with a remote central monitoring station via one or more network interface units connected to a private or public wired or wireless network.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation-in-part of application Ser. No. 08/931,089, filedSep. 15, 1997, now U.S. Pat. No. 6,058,137, entitled “FREQUENCY HOPPINGSYSTEM FOR INTERMITTENT TRANSMISSION” which application is incorporatedby reference.

FIELD OF THE INVENTION

The present invention relates to telemetry in general and, moreparticularly, to a plurality of cooperating telemetry systems.

BACKGROUND OF THE INVENTION

In certain types of radio systems there exist many battery operatedtransmitters that periodically transmit short duration messages to oneor more receivers. One example of such systems are telemetry systems,another example is burglary and fire alarm systems that can also beviewed as a specific telemetry system. In these systems, manytransmitters located at different places transmit messages indicative ofthe status of monitoring sensors to a telemetry receiving station (e.g.,a receiver with a processor that collects the data from the sensors,etc.). Usually the transmitters include a battery status and sometimesalso the temperature in the transmitted messages in addition to themonitored sensor status. Normally, the transmitters transmit supervisorystatus messages that are as short as feasible and the period between thetransmissions is as long as feasible in order to minimize the averagecurrent drain from the battery. In addition, short and infrequenttransmissions lower the probability that the data is lost due tocollisions which occur when two or more transmitters transmit at thesame time. However, when an alarm or an abnormal condition occurs, atransmitter transmits immediately in order to convey the alarm messagewith little delay.

In order to identify the sensors, each associated transmitter isnumbered and identified by a transmitter identification number(identification) number. Usually, such a system has one telemetryreceiving station that receives data from the transmitters. Typically,the telemetry receiving station has to convey unusual system status to acentral monitoring facility that collects data from several systems thatare often placed in different geographical locations.

SUMMARY OF THE INVENTION

The illustrative embodiment of the present invention comprises aplurality of telemetry systems where each system comprises severalremote telemetry transmitters to intermittently transmit short durationmessages indicative of the status of the sensors associated with thetransmitters, and at least one telemetry collection unit having a radiotransceiver. In an illustrative embodiment of the present invention, thetelemetry collection units forward alarm messages received from othertelemetry collection units and respond to messages from remote telemetrytransmitters associated with other telemetry collection units. Ineffect, the telemetry collection units and the associated remotetelemetry transmitters form a meshed wireless telemetry system in whichtelemetry collection units cooperate with each other in order toincrease reliability and lower the cost of each system. Such meshedtelemetry system can communicate with a remote central monitoringstation via one or more network interface units connected to a privateor public wired or wireless network. In an illustrative embodiment, thenetwork interface unit is wireless equipment that connects to onetelemetry collection unit. In accordance with an illustrativeembodiment, there are fewer wireless network interface units than thereare telemetry collection units in the meshed system, thus, in a meshedsystem, the cost of the wireless network interface unit can be sharedbetween several telemetry systems. However, in an alternative embodimentthe network interface unit is a wireline modem, and there may be onemodem for each telemetry system.

In accordance with an illustrative embodiment, the telemetry system hasthe telemetry collection unit receiver augmented by a transmittercircuitry thus rendering it to become a system transceiver. Such amodification is simple and inexpensive since the transmitter can sharemost of its circuits with receiver. For example the transmitter controlcan be accomplished by the same logic circuit or microprocessor as thereceiver. Also some of the radio frequency circuits such as thesynthesizer and reference frequency can be reused.

In accordance with an illustrative embodiment, the telemetry collectionunit receives messages from the remote telemetry transmitters that areassociated with this telemetry collection unit and it is responsive tothe received messages. In addition, the telemetry collection unitreceives and is responsive to the messages transmitted by remotetelemetry transmitters associated with other telemetry collection units.In an illustrative embodiment, the telemetry collection unit has theidentification numbers of the transmitters associated with thistelemetry collection unit. In addition, in the illustrative embodiment,in operation, the telemetry collection unit is furnished withidentification numbers of remote telemetry transmitters that areassociated with other telemetry collection units and that are to bemonitored by this telemetry collection unit. In the illustrativeembodiment, such furnishing is accomplished by exchanging informationand cooperation with the other telemetry collection units.

In an illustrative embodiment, the telemetry collection units fromdifferent systems communicate with each other and exchange informationto obtain mutual coverage of their respective transmitters. Inoperation, telemetry collection units of two systems establishcommunications with each other and exchange information about the remotetelemetry transmitters that belong to each system. This may includetransmitter identification and signal quality of the received messages.Based on the exchanged information, the telemetry collection units canagree to receive and be responsive to the messages received from theremote telemetry transmitters that belong to the other system.

In accordance with an illustrative embodiment, the telemetry collectionunits from different systems communicate with each other and exchangeinformation in order to pass along alarm (or other) messages until theyreach a terminating node, i e. a wired or wireless network interfaceunit or a telemetry collection unit that is coupled to or equipped witha wired or wireless network interface unit. In accordance with anillustrative embodiment there are fewer network interface units thanthere are telemetry collection units, thus making it possible to shareone network interface unit between several systems. However, in analternative illustrative embodiment there is one network interface unitfor each telemetry system. E.g. in a burglary alarm application, eachsystem is equipped with a wireline modem that is vulnerable to an attackby cutting the phone line. In such case, the telemetry collection unitof the system under attack can sent an alarm information to a centralmonitoring facility by communicating with another telemetry collectionunit that belongs to another system. This way reliability is improvedwithout an expense of a wireless interface unit.

In accordance with an illustrative embodiment, in operation, thetelemetry collection unit (receiver) receives messages from the variousremote telemetry transmitters, some of which may require that thecentral monitoring facility be notified. When such a message isreceived, the telemetry collection unit transmits a message that isdirected to a telemetry collection unit from other system. Ordinarily,this is the nearest telemetry collection unit. Upon reception andconfirmation, the message is passed along in a similar way to the nexttelemetry collection unit. If the confirmation is not received, themessage may be retransmitted a predetermined number of times. Similarly,the message is passed from one telemetry collection unit to the nextuntil it reaches the final destination that is: a wired or wirelessnetwork interface unit or a telemetry collection unit coupled to orequipped with a wired or wireless network interface unit. In effect, thetelemetry collection units form nodes in a network through which themessages are routed from the source to the destination,i.e. theterminating node. At any point along the way the message is routed, thepath selected by a telemetry collection unit can be altered as needed,e.g. if the nearest telemetry collection unit is not operative, analternative route can be selected. Preferably, the alternative paths areselected on the basis of the signal quality and distance to theterminating node, wherein the signal quality is measured along theentire selected path to maximize the transmission reliability.

Similarly, in accordance with an illustrative embodiment messages arecarried in reverse direction; i e. from network interface unit to atelemetry collection unit.

In accordance with an illustrative embodiment, in some cases it may bepreferable that the messages are broadcasted (rebroadcasted) instead ofrouted. In such a case, the originating node broadcasts the firstmessage, then each telemetry collection unit that received the messageretransmits (rebroadcasts) the message, thus making it possible for themessage to reach many nodes of the system. In a way, the message floodsthe network thus ensuring that at least one terminating node receivesthe message. The message may be rebroadcasted by each node apredetermined number of times, or alternatively, rebroadcasting stopswhen the confirmation message is received from the terminating node.

Similarly, in accordance with an illustrative embodiment messages arecarried in reverse direction; i.e. from network interface unit to atelemetry collection unit.

In accordance with an illustrative embodiment, each remote telemetrytransmitter transmits short duration messages at predetermined timeintervals in such a way that each transmitter transmits each consecutivemessage at a different frequency. For each remote telemetry transmitter,the frequency variations are selected according to a sequence, and thesequence is determined individually for each transmitter based on thetransmitter identification number. The frequency sequences areorthogonal, a coincidence of frequencies at one time excludes thecoincidence at any other time for the duration of the entire sequence.This way, the possibility is eliminated that two or more remotetelemetry transmitters interfere with each other during more than onemessage transmission for the duration of the entire sequence. Inaddition, transmission at varied, i e. diverse, frequencies reducesprobability that all transmitted messages are lost due to interferenceor signal fading that are predominantly frequency selective.

Alternatively, in accordance with an illustrative embodiment, eachremote telemetry transmitter can vary the time betweentransmissions—TBT—according to a predetermined pattern. Preferably, foreach remote telemetry transmitter, the pattern of variations depends onthe transmitter identification number. This way, the possibility iseliminated that two or more remote telemetry transmitters interfere witheach other during more than one message transmission for the duration ofthe entire sequence.

In accordance with another illustrative embodiment, the remote telemetrytransmitters vary both the frequency and time between transmissions inorder to maximize the system reliability. In this way, the receiver isrelived from the burden to receive more than one message at differentfrequencies at the same time. Thus, the receiver circuit complexity isreduced. Preferably, the frequency-time sequences are orthogonal.

In accordance with an illustrative embodiment, the transmitters in thetelemetry collection units perform similar periodic transmissions andfrequency and time variations as the remote telemetry transmitters asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a block diagram of a plurality of telemetry systems in theprior art.

FIG. 1b is a block diagram of two illustrative telemetry systems in theprior art.

FIG. 2a is a block diagram of plurality of telemetry systems inaccordance with the illustrative embodiment of the present invention.

FIG. 2b is a block diagram of an illustrative embodiment of the presentinvention.

FIG. 2c is a block diagram of another illustrative embodiment of thepresent invention.

FIG. 3 is a diagram of a plurality of telemetry systems working as anetwork.

FIG. 4 is a diagram of the preferred remote telemetry transmitterimplementation.

FIG. 5 is a diagram of the preferred receiver implementation.

FIG. 6 is a diagram of the PN generator used to affect the frequency andtime changes in the preferred remote telemetry transmitters and in thetelemetry collection units.

FIG. 7 is a diagram of frequency hopping transmissions includingtransmission opportunities for transmitting urgent messages inaccordance with the illustrative implementation of the presentinvention.

DETAILED DESCRIPTION

FIG. 1a depicts a plurality of illustrative telemetry systems in theprior art. Each of the telemetry systems 100-1, 100-2, 100-3, etc, to100-N interfaces independently with a central monitoring facility 160over a telecommunications network 150. Each of the telemetry systems andthe central monitoring facility are, typically, located in differentgeographical locations.

FIG. 1b depicts two illustrative telemetry systems in the prior art,telemetry system 100-1 and telemetry system 100-i, that are bothmonitored by central monitoring facility 160. Telemetry system 100-1comprises:

(i) one or more remote telemetry transmitters (e.g., remote telemetrytransmitters 121-1-1, 121-1-2, and 121-3, etc.) comprising radiotransmitters 101-1-1, 101-1-2, and 101-1-3, etc. that are each fedinformation by an associated sensor (e.g., monitoring sensors 111-1-1,111-1-2, and 111-1-3, etc.);

(ii) telemetry receiving station 130-1, which comprises a radio receiver(e.g., radio receiver 131-1, etc.) that receives the telemetrytransmitted by the remote telemetry transmitters; and

(iii) one or more keypads 133-1 for enabling a user to enter commandsinto telemetry receiving station 130-1,

(iv) one or more displays 132-1 for providing information to a user fromtelemetry receiving station 130-1;

(v) an electro-acoustic transducer (e.g., a loudspeaker, a bell, etc.)134-1 for alerting a user of an alarm; and

(vi) network interface unit 140-1 that enables telemetry, commands, andother signals to pass between telemetry receiving station 130-1 andcentral monitoring facility 160, via network 150 (e.g., the publicswitched telephone network, the Internet, etc.). The composition oftelemetry system 100-i is analogous to that of telemetry system 100-1.Telemetry receiving station 130-1 advantageously has associated with it:(1) one or more keypads for local command entry, (2) one or moredisplays for providing information, and (3) an electro-acoustictransducer (e.g., a loudspeaker, a bell, etc.) for alerting a user of analarm.

In operation, each remote telemetry transmitter periodically orsporadically transmits telemetry to its associated telemetry receivingstation, which telemetry typically comprises: sensor status, batterystatus, temperature, message number, etc. When a remote telemetrytransmitter indicates that its associated sensor detects an alarmcondition, the telemetry receiving station communicates that fact tonetwork interface unit 140-1, which, in turn, forwards that informationto the central monitoring facility 160. Conversely, central monitoringfacility 160 can initiate communications with the telemetry receivingstations to: (i) learn the status of the remote telemetry transmittersor the telemetry receiving station, and (ii) to change the telemetryreceiving station state such as the alarm thresholds for the monitoredsensors.

In accordance with FIG. 1b, telemetry system 100-1 and telemetry system100-i operate independently (i.e., the telemetry receiving station ofone telemetry system is not responsive to the telemetry transmitted bythe remote telemetry transmitters that are associated with anothertelemetry system). Furthermore, both telemetry system 100-1 andtelemetry system 100-i have their own network interface unit with whichto communicate with the central monitoring facility.

Typically, the cost of a network interface unit far exceeds the cost ofthe telemetry receiving station, which increases the overall cost of thetelemetry system significantly. Furthermore, when a wireline connectionto network 150 is used, an additional problem arises if there is nowireline access where the telemetry receiving station is to be located.Although a wireless network interface unit solution might solve accessproblems, such units tend to be expensive.

The reliability of telemetry systems 100-1 and 100-i is also in issue.For example, the failure of the network interface unit or the telephoneline between the network interface unit and network 150 renders thetelemetry system useless. Furthermore, even a failure of a telemetryreceiving station renders the system useless.

FIG. 2a depicts a block diagram of a plurality of telemetry systemsaccording to an illustrative embodiment of the present invention. Eachof the telemetry systems and the central monitoring facility are,typically, located in different geographical locations, although theymay be in sufficient proximity such that a telemetry system cancommunicate with some of its neighbors. Each of the telemetry systems200-1, 200-2, 200-3, etc, to 200-N can communicate with a centralmonitoring facility 160 over a telecommunications network 150. However,the communications is accomplished via indirect routs using othertelemetry systems as nodes in a network. Only selected telemetry systemsare equipped with network interface units. Each telemetry system cancommunicate with at least one other telemetry system. Thus, for eachtelemetry system, there exists at last one path to reach one or morenetwork interface units. In an advantageous implementation, there existat least two alternative paths that do no not include a node common forboth paths.

FIG. 2b depicts a block diagram of the illustrative embodiment of thepresent invention, which comprises: telemetry system 200-1, telemetrysystem 200-i, network 150 and central monitoring station 160. Telemetrysystem 200-1 comprises:

(i) one or more remote telemetry transmitters (e.g., remote telemetrytransmitters 221-1-1, 221-1-2, and 221-3, etc.) comprising radiotransmitters 201-1-1, 201-1-2, and 201-1-3, etc. that are each fedinformation by an associated sensor (e.g., monitoring sensors 211-1-1,211-1-2, and 211-1-3, etc.);

(ii) telemetry collection unit 230-1, which comprises a radiotransmitter 232-1, a radio receiver (e.g., radio receiver 231-1, etc.)that receives the telemetry transmitted by remote telemetry transmitters221-1, 222-1, and 223-1, at least one of the remote telemetrytransmitters associated with telemetry system 200-i, and messages fromtelemetry collection unit 230-i; and

(iii) one or more keypads 233-1 for enabling a user to enter commandsinto telemetry collection unit 230-1,

(iv) one or more displays 232-1 for providing information to a user fromtelemetry collection unit 230-1;

(v) an electro-acoustic transducer (e.g., a loudspeaker, a bell, etc.)for alerting a user of an alarm; and

(vi) network interface unit 240-1 that enables telemetry, commands, andother signals to pass between telemetry collection unit 230-1 andcentral monitoring facility 160, via network 150 (e.g., the publicswitched telephone network, the Internet, etc.).

The composition of telemetry system 200-i is analogous to that oftelemetry system 200-1 except that it does not comprise a networkinterface.

Although the network interface unit 240-1 is shown as a separate entity,it is to be understood that in some applications, such network interfaceunit can be an integral part of the telemetry collection unit 230-1. Inany event it can be said that the telemetry collection unit isoperatively coupled to the network interface unit.

Although only two telemetry systems are depicted in FIG. 2b, someembodiments of the present invention will have two or more telemetrysystems that cooperate and communicate amongst themselves. In theillustrative embodiment of the present invention, each of telemetrycollection unit 230-1 and telemetry collection unit 230-i can establishpeer-to-peer communications and exchange information about theirrespective systems.

For example, when the telemetry collection unit 200-1 receives telemetryfrom one or more remote telemetry transmitters associated with telemetrysystem 200-i that indicate an alarm condition, telemetry collection unit200-1 can confer with telemetry collection unit 200-i on whethertelemetry collection unit 200-1 should send an alarm to centralmonitoring facility 160 via network interface unit 240. The fact thatthe telemetry collection units can communicate with each other enablesproximate telemetry systems to share one or more network interfaceunits, which drops the average cost of a telemetry system.Alternatively, each telemetry system can be equipped with a lessreliable network interface unit (e.g. wireline modem). Thecommunications between the telemetry collection units increasesreliability, since an alarm can be transmitted to a central monitoringfacility even if the wireline is cut-off from one the telemetry systems.

In addition, each remote telemetry transmitter has a uniqueidentification number associated with it, and the information exchangedbetween the telemetry collection units includes the identificationnumbers of their associated remote telemetry transmitters. For example,telemetry collection unit 230-1 informs telemetry collection unit 230-iof the identification numbers of the remote telemetry transmittersassociated with telemetry system 200-1, and vice versa. Therefore, iftwo or more telemetry collection units are in proximity and the signalquality of the telemetry from the other's remote telemetry transmittersis satisfactory, then the telemetry collection units can agree tomutually monitor the telemetry from their aggregate remote telemetrytransmitters.

In operation, each remote telemetry transmitter intermittently transmitsshort telemetry messages that advantageously comprise: informationindicative of sensor status (e.g. switch closure status), batterystatus, temperature, etc.

Each telemetry collection unit monitors all the remote telemetrytransmitters that belong to the same system as well as the remotetelemetry transmitters that belong to other systems with which thetelemetry collection unit has an agreement for a mutual coverage. When atelemetry collection unit receives an alarm message from a remotetelemetry transmitter that belongs to another system, the telemetrycollection unit is responsive to that message by first communicatingwith the telemetry collection unit in the originating system. When thetelemetry collection unit in the originating system is not operative,the telemetry collection unit establishes communications and sendsappropriate information to the central monitoring facility via thenetwork interface unit. If the telemetry collection unit is notconnected to a wireless network interface unit, it passes an appropriateinformation to another telemetry collection unit. It should be apparentthat this way the system is not vulnerable to a failure or intentionaldestruction of any single telemetry collection unit. In effect, thequality and reliability of all the systems is improved and expense islowered.

FIG. 2c illustrates another exemplary embodiment of the presentinvention that comprises: telemetry collection unit 234-1 withassociated remote telemetry transmitters 221-1-1, 221-1-2, 221-1-3,telemetry collection unit 234-2 with associated remote telemetrytransmitters 221-2-1, 221-2-2, 221-2-3, and telemetry collection unit234-3 with associated remote telemetry transmitters 221-3-1, 221-3-2,and 221-3-3. Each telemetry collection unit comprises a receiver 231-1,231-2, and 231-3 respectively, a transmitter 232-1, 232-2, and 232-3respectively. In addition, each telemetry collection unit comprisesembedded modem that serves as a network interface unit 241-1, 241-2, and241-3 respectively.

In operation, each telemetry collection unit receives telemetry from itsassociated remote telemetry transmitters and, in addition, from someremote telemetry transmitters that are associated with other telemetrycollection units, e.g. over paths 271 to 276.

In operation, the telemetry collection units communicate via wirelesspaths 277, 278 and 279 respectively to exchange identification codes ofthe associated remote telemetry transmitters and to communicate alarmsthat originate in the remote telemetry transmitters.

It should be apparent that if a network interface unit or even atelemetry collection unit is inoperative or cut-off from the publicnetwork, an alarm can be detected by another telemetry collection unitand sent over the public network to a central monitoring facility. Also,if the phone line is cut-off, the telemetry collection unit can detectthe condition and immediately send a warning message to the centralmonitoring facility. Thus, using a low cost wireline modems, the systemcan provide high reliability and immunity from an attack by anintentional cutting off the telephone line.

In the advantageous embodiment, in operation, the telemetry collectionunits manage passing the alarm messages and communications with thecentral monitoring facility in a coordinated way.

Referring to FIG. 3, several telemetry systems constitute a network ofsystems. In FIG. 3, the systems are represented by their telemetrycollection units: 231-1, 231-2, 231-3, 231-4, 231-5, and 231-6. In somesystems,i.e. 231-1 and 231-5 the telemetry collection units areconnected with network interface units 240-1 and 240-5. Depending on theproximity and radio propagation conditions, the telemetry collectionunits can communicate with each other over the radio paths 381, 382,383, 384, 385, 386 and 387. For example, in FIG. 3, telemetry collectionunits 231-3 can communicate with telemetry collection units 231-2 and231-6; telemetry collection unit 231-6 can communicate with telemetrycollection units 231-3, 231-4, and 231-5; telemetry collection unit231-5 can communicate with telemetry collection units 231-6 and 231-4,etc. It is apparent, that each telemetry collection unit can communicatewith one or more other telemetry collection units but not all othertelemetry collection units.

In operation, a telemetry collection unit that receives an alarm messagefrom a remote telemetry transmitter that belongs to the same systemoriginates and transmits an alarm message. The alarm message ispreferably directed (addressed) to another (neighboring) telemetrycollection unit. The neighboring telemetry collection unit retransmitsthe received alarm message directing it to another telemetry collectionunit. The process is repeated until the message is passed to a telemetrycollection unit that interfaces to a network interface unit. Suchtelemetry collection unit is a terminating telemetry collection unitbecause it does not pass along the message to another telemetrycollection unit. Instead, the terminating telemetry collection unitsends appropriate information to the central monitoring facility via thenetwork interface unit. In order to facilitate orderly retransmission ofthe alarm message by various telemetry collection units (nodes of thenetwork), the telemetry collection units route the message according toa predetermined algorithm and according to a routing table that containsinformation about the number of hops and relative signal quality alongthe chosen message path. The number of hops indicates the number ofretransmissions needed to reach the terminating telemetry collectionunit, and the relative signal quality is a composite quality along theentire message path determining likelihood for successful message travelalong the chosen path. For example, the telemetry collection unit 231-3has a table with two entries related to two possible paths beginningwith telemetry collection units 231-2 and 231-6. The number of hops andthe signal quality is determined as the best of the possible paths. Forexample, the telemetry collection unit 231-2 can transmit the alarmmessage to the terminating node 231-1 directly or via node 231-4. Tocompute the information for the path along telemetry collection unit231-2 for the table of the telemetry collection units 231-3, the best ofthe two paths from the telemetry collection units 231-2 to a terminatingsource is taken. Similarly, the telemetry collection unit 231-6 can sendthe alarm message to the terminating node 231-5 directly or via node231-4. Again the best path information is taken to compute the pathalong telemetry collection unit 231-6 for the table of the telemetrycollection units 231-3.

In the advantageous embodiment, in order to collect the necessaryinformation in the system nodes, the terminating telemetry collectionunits periodically transmit predetermined short duration messages orprobes. The probes are retransmitted by each network node. Eachretransmitted probe contains the number of hops the message traveled sofar and the composite signal quality. At each node this information isupdated before retransmission. The probe messages also contain theidentification number of the last retransmitting node. Each nodecollects the statistics of the received probes and their pathinformation. By way of comparison and elimination, the path quality isestablished for all paths at each node. In operation, if the best pathis not operative, e.g. when the neighboring telemetry collection unit ismalfunctioning, an alternative path can be selected at each node of thenetwork. Thus, the message that leaves the originating node is alwaysensured to reach the terminating node. The path information update mayrequire some time and may also require significant network traffic ifthe probes are sent often. Therefore, the compromise is required whichmay result in some tables to be momentarily outdated. In some situationswhen the highest priority alarm messages are to be sent, a simpler butmore robust algorithm is preferred.

Accordingly, in the advantageous embodiment, the originating nodebroadcasts the priority alarm message instead of directing it to aspecific node. Each node that successfully receives the priority alarmmessage rebroadcasts the message. The rebroadcasting is continued byeach new node that receives the priority alarm message. In effect, themessage “floods” the entire network to eventually reach one or moreterminating nodes. Each terminating node upon reception of the priorityalarm message, independently informs the central monitoring facility viaits modem or wireless network interface unit. Each node rebroadcasts thepriority alarm message a predetermined number of times or until aconfirmation message is received from a terminating node that receivedthe broadcasted priority alarm message and successfully informed themonitoring facility. The confirmation message is propagated through thenodes of the network in a way similar to the priority alarm message.I.e., the terminating node brodcasts the confirmation a predeterminednumber of times and each node that received the message rebroadcasts themessage a predetermined number of times. In order to make the process ofrebroadcasting orderly, each priority alarm message has anidentification number that identifies the message. Preferably, this isthe identification number of the originating remote telemetrytransmitter augmented with an identification of the telemetry collectionunit and a time stamp. Each node that receives the priority alarmmessage stores the message identification for a predetermined timeduring which all alarm message with the same identification are ignored.Thus, the process of rebroadcasting terminates after each noderebroadcasts the message a predetermined number of times.

It should be apparent that the same methods of transmission (i.e. routedor broadcasted) can be used to carry messages in the opposite direction,i.e. from a network interface unit to a selected telemetry collectionunit. Such messages may be sent when a central monitoring facility needsto inquire about the status of a selected telemetry collection unit.

It should now be apparent that the combined features of the mutualcoverage of the remote telemetry transmitters of two or more telemetrycollection units and the capability of the telemetry collection units toact in a specific way as nodes of a network which can carry alarmmessages across, makes the telemetry system more reliable, morefunctional and less expensive.

In the advantageous embodiment, all remote telemetry transmitterstransmit periodically short duration messages at predetermined timeintervals. The transmission time for each remote telemetry transmitteris not coordinated between different remote telemetry transmitters.Consequently, sometimes the messages from different remote telemetrytransmitters may happen to coincide and interfere with each other. Tomitigate this effect, in the preferred embodiment, each remote telemetrytransmitter varies the transmission frequency for each consecutivemessage. The frequency variations are large enough so that messages canbe separately received at different frequencies without interference.The frequency variations are done individually for each remote telemetrytransmitter according to predetermined patterns that are, preferably,based on the transmitter identification number as described later indetails.

Preferably all sequences are orthogonal. I.e. for any two transmitters,a coincidence of frequencies at one time excludes the coincidence at anyother time for the duration of the entire sequence. This way, thepossibility is eliminated that two or more transmitters interfere witheach other during more than one message transmission for the duration ofthe entire sequence. In addition, transmission at varied (i.e. diverse)frequencies reduces probability that all transmitted messages are lostdue to interference or signal fading that are predominantly frequencyselective.

Alternatively, each remote telemetry transmitter varies the time betweentransmission—TBT—according to a predetermined pattern. Preferably, foreach remote telemetry transmitter, the pattern of variations depends onthe transmitter identification number. This way, the possibility iseliminated that two or more transmitters interfere with each otherduring more than one message transmission for the duration of the entiresequence.

In the advantageous embodiment, the remote telemetry transmitters varyboth the frequency and time between transmissions in order to maximizethe system reliability. One advantage of the additional (time) variationis that, this way, the receiver is relived from the burden to receivemore than one message at different frequencies at the same time. Such aneed could arise if two remote telemetry transmitters transmit at thesame time (albeit at different frequencies). Thus, the receiver circuitcomplexity is reduced and the system reliability is improved.

Referring to FIG. 4, in accordance with an illustrative embodiment ofthe present invention, the remote telemetry transmitter 221-i-j includesa radio transmitter (transmitter) 210-i-j including a referencefrequency crystal oscillator 406 to produce a stable frequency on line426, a time interval generator 402 establishing a time base to producepulses on line 428 activating the transmitter, a frequencysynthesizer-modulator 404 to produce a radio frequency carrier modulatedby modulation data fed to the synthesizer via line 424 wherein thefrequency of the carrier is programmed to a desired value via pluralityof lines 414, transmitter control logic 408 to activate and program thesynthesizer-modulator 404 via plurality of lines 414 when the logic isactivated by a pulse from the time interval generator or by an abnormalsignal indication on a sensor signal input line 418, an amplifier 410 toamplify the radio carrier provided by the synthesizer when the amplifieris activated by the control logic 408 via line 416, and an antenna 412to radiate the power delivered by the amplifier. The control logic 408includes a frequency and time data memory register 420 to holdinformation used to determine the time and the frequency of nexttransmission, and a sensor interface circuit 422 to accept the sensorsignal and detect an abnormal signal condition, and to convert thesensor signal to a digital format suitable for transmission. Thetransmitter logic also includes a storage means 430 to store atransmitter identification number to differentiate this transmitter fromother transmitters. The transmitter control logic, in some systems, canbe realized based on a microprocessor, in some other systems, aspecialized component may be used. In an illustrative implementation,the remote telemetry transmitter includes also one or more sensors211-i-j responsive to changes that are to be monitored and to produceappropriate signals at the interface 418.

In operation, during the time between transmissions, the transmitter isin a standby mode in which the amplifier 410 and synthesizer-modulator404 are not active and, preferably, the control signals turn off thepower from these circuits in order to minimize the standby current ofthe transmitter. The transmitter control logic 408 is in a standby modein which most of the circuits are inactive and some or most of thecircuitry can be powered down with the exception of the circuitssupporting critical functions; (a) the sensor interface circuit 422 thatdetects an abnormal signal condition and produces a binary signal thatis logically combined with the signal 428 produced by the time intervalgenerator so that when either a pulse or abnormal condition occurs therest of the transmit logic circuitry is activated or powered up, (b) thefrequency and time data memory 420 that has to retain the data duringthe period between transmission and consequently either it has to be anonvolatile type or it has to be powered up during the period betweentransmissions. Upon activation, the control logic 408 determines theactivation source by reading signals 428 and 418.

When the logic 408 is activated by a pulse 428 from the time intervalgenerator the following sequence of events occurs. First, the logicreads the frequency data memory and produces a data packet that includesthe sensor status, the transmitter identification number and other datasuch as battery status. Then, the logic activates and programs thesynthesizer-modulator 404, activates the amplifier 410 and sends thepacket to the modulator via line 424. After completion of eachtransmission, the transmitter logic sets the transmitter in the standbymode until activated again by a pulse on line 428 or a sensor abnormalcondition indicated on line 418.

In an advantageous embodiment the transmission of a packet can berepeated a predetermined number of times at separate frequencies,wherein the number of repetitions is chosen according to applicationneeds and, wherein the frequencies are determined by the transmitterlogic according to an algorithm described later in details. This way, itis possible for the receiver to receive some repeated packets even ifother packets are lost due to frequency selective fading caused bymultipath or due to interference.

When a sensor abnormal condition occurs, the sensor interface circuit422 produces an active level of the signal indicative of the sensorabnormal level which activates the transmitter via a combinatorial logiccircuit that combines the sensor abnormal level signal with the pulsesfrom the time interval generator. When activated this way, thetransmitter control logic 408 produces a data packet that includes thesensor status, then the logic activates and programs thesynthesizer-modulator 404, activates the amplifier 410, and sends thepacket to the synthesizer-modulator. In an advantageous embodiment, thetransmission of the alarm packet is repeated a predetermined number oftimes using a plurality of predetermined alarm frequencies in such a waythat the transmission frequency is changed after each single packettransmission according to a predetermined fixed sequence. The essence ofthe idea is that the alarm message being infrequent can afford a muchgreater transmission overhead and can be repeated several times. In someapplications, the transmissions of alarm packets are advantageouslyperformed at “transmission opportunities” in accordance with a methoddescribed later in details.

After the transmission sequence is completed, the control logic disablesthe signal indicative of the sensor abnormal status so that an abnormalsensor status can not activate the control logic. Then, the controllogic puts the transmitter in the standby mode until activated by apulse from the time interval generator. When subsequently activated, thetransmitter control logic performs the usual transmission sequence butthe data packets include information that the sensor condition isabnormal if the condition persists. When the abnormal conditionsubsides, the signal indicative of an abnormal status is enabled so thata subsequent occurrence of an abnormal condition can activate the logicand trigger a new alarm transmission sequence; thus, normal operation isrestored.

In an advantageous embodiment, the sequence in which these frequenciesare used is determined individually for each transmitter. The followingis the description how this is accomplished in the advantageousembodiment. Each transmitter includes a pseudo random sequencegenerator, wherein a pseudo random sequence generator is based on alinear feedback shift register, wherein some outputs of the shiftregister are fed back to an EX-OR (Exclusive OR) gate whose output isconnected to the register input. For a certain combination of theoutputs that are fed to the EX-OR gate, the shift register can produce asequence that has 2^(N)−1 bits, wherein N is the length of the shiftregister. Such a sequence is called a maximum length sequence.Alternatively, if all the outputs of the shift register are taken at atime, then a pseudo random sequence of 2^(N)−1 numbers is created,wherein all the numbers are N digits long and each number differs fromall the other numbers in the sequence; the numbers range from 1 to2^(N)−1. Such pseudo random generators are known to the skilled in theart. For example, a three-bit PN generator is based on a three-bit shiftregister with feedback taken from the first and the last bit. Thisregister produces a sequence of seven numbers, wherein each number hasthree digits. The numbers change from 1 to 7.

Referring to FIG. 6, the pseudo random sequence generator 603 consistsof a shift register 605 and EX-OR gate 604. The shift register 605 iscomposed of three stages 621, 622, and 623 having three outputs Q₀ 611,Q₁ 612 and Q₂ 613 respectively. The feedback is taken from outputs Q₀and Q₂. The three least significant bits of the transmitteridentification {i₂, i₁, i₀} 601 are combined with the output of thepseudo random sequence generator {Q₂, Q₁, Q₀} using EX-OR gates 608,607, 606. The result can be used to indicate the frequency or frequencychannel index {f₂, f₁, f₀} 602 over which the transmission will occur.

Assuming that the initial state of the shift register is binary 111(decimal 7), the produced sequence is {7, 3, 5, 2, 1, 4, 6}. Thesenumbers are then combined with the last three bits of the transmitteridentification using bit by bit EX-OR operation; i.e. the last bit ofthe transmitter identification (i₀) is combined with the last bit of therandom number (Q₀), etc. This way produced new sequence has numbersranging from 0 to 7 the order of which depends on the last three bits ofthe transmitter identification. Thus, 8 distinct (permutated) sequencesof numbers are created. These sequences are used to select thetransmission frequencies. For example, if the last digits of thetransmitter identification are 000, then the frequencies are selected inthe order 7, 3, 5, 2, 1, 4, 6,i.e. the sequence is not altered. If thelast three digits of the transmitter identification are 001, then thefrequencies are selected in the order 6, 2, 4, 3, 0, 5, 7; if the lastthree digits of the transmitter identification are 010, then thefrequencies are selected in the order 5, 1, 7, 0, 3, 6, 4; etc. Iflonger shift register is used, longer sequences are generated using morefrequencies. In the preferred embodiment, an 8-bit shift is used toproduce sequences that use 256 frequencies. Each number in the sequencerepresents an index based on which the actual frequency is determined.

Similarly, according to an exemplary implementation, the time betweenthe transmissions is randomized using the transmitter bits.Advantageously, the transmitter bits used for time randomization aredifferent from the bits used for the time randomization. However,advantageously, the same PN generator outputs are utilized, thus makingit easier to synchronize the receiver with the time variations.

In determining the time between the transmissions, the produced indexesare used to determine the increments or decrements of the nominal timebetween the transmissions. Advantageously, according to an exemplaryimplementation, the transmitter control logic 408 (FIG. 4) programs thetime interval generator via lines 427 in order to change the timerperiod accordingly.

The receiver of the telemetry collection unit has to synchronize andtrack the timing and frequency of each monitored remote telemetrytransmitter.

Referring to FIG. 5, the receiver includes a reference frequency crystaloscillator 526 to produce a stable reference frequency on line 528 forthe receiver circuits, a frequency selective radio receiver circuit 500whose frequency is programmable via lines 516, to receive and demodulatea frequency modulated carrier when the frequency of the frequencyselective receiver circuit is programmed according to the frequency ofthe carrier, and a receiver control logic means 530 to processdemodulated data, to provide system interface lines 540, responsive tothe received data, and to program the frequency of the frequencyselective receiver circuit. The control logic includes a receiver timer532 establishing a time base to measure the elapsing time. The controllogic also includes: (a) a plurality of identification memory registers534 to hold digital data indicative of identification numbers for eachremote telemetry transmitter that belongs to the system, (b) a pluralityof time memory registers 536 to hold digital data indicative of the timeof the next transmission occurrence for each respective remote telemetrytransmitter, and (c) a plurality of frequency memory registers 538 tohold digital data indicative of the frequency of the next transmissionoccurrence for each respective remote telemetry transmitter. In theillustrative embodiment, the registers are organized such that if anarbitrary register i 551 of the plurality of identification memoryregister 534 contains a transmitter identification number n, thenregister i 552 of the plurality of time memory registers 536, andregister i 553 of the plurality of frequency memory registers 538 arealso associated with the same transmitter whose identification number isn. The frequency selective radio receiver circuit 500 includes a RFband-pass filter 504, an amplifier 506, an IF band-pass filter 510, amixer 508, limiter-discriminator circuit 512 and frequency synthesizer514. The RF band-pass filter selects only the desired frequency bandallocated for the transmission, the mixer mixes the incoming signal withthe signal produced in the frequency synthesizer and produces an IFfrequency (Intermediate Frequency). The IF frequency is filtered in anarrow band filter 510 whose bandwidth is selected according to thechannel bandwidth. The limiter discriminator demodulates the signal andproduces base-band DATA signal 520 and an RSSI signal 518 indicative ofthe received signal strength. The DATA signal 520 and the RSSI signal518 are converted to binary signals by A/D converters 524 and 522respectively and fed to the control logic 530. The presentedarchitecture of the frequency selective radio receiver circuit 500 isknown as a super-heterodyne FM receiver; it is well known and it doesnot require additional explanation. The transmitted message data isextracted from the DATA signal 520 digitized by the A/D converter 524using one of the many well-known methods for signal processing and doesnot require additional explanation.

In the advantageous embodiment, the frequency registers 538 hold foreach transmitter the state of PN generator. If the synchronization isobtained with a given transmitter, the state of the PN generator isidentical with that in the transmitter.

In the advantageous embodiment, the time registers 536 hold numbers—timeof next transmission—for each transmitter representing the state of thereceiver timer 532 at the time the next transmission is due from atransmitter.

In operation, the receiver control logic 530 sequentially compares thedata content of the time registers 536 with the data content of thereceiver timer 532 and if the transmission is due from a remotetelemetry transmitter n whose time data is stored in register i 552, thecontrol logic programs the frequency selective radio receiver circuit500 according to the content of the frequency register i 553 and theidentification register i 551 that are associated with the sametransmitter n, attempts to decode the demodulated signal, changes thecontent of the time register based on the number representative of thetime interval between the transmissions for this transmitter and changesthe content of the frequency register according to a predeterminedalgorithm for this transmitter. I.e. the frequency and the timeregisters are updated each time a transmission is due regardless whetherthe transmission was received successfully. The new content of thefrequency register is determined according to the algorithm for thefrequency use by the transmitters.

Nominally, the time between transmission—TBT—is the same for all remotetelemetry transmitters but due to unavoidable tolerances and temperaturevariations, the TBT may be slightly different for different transmittersand it can vary with time. Therefore, the new content of the timeregister is calculated based on the current content of the receivertimer and a number representative of the time between the currenttransmission and the next transmission for this transmitter, where thisnumber is calculated based on the nominal value of the time between thetransmissions and adjusted by a correction factor that is based on themeasured difference between the transmitter time base and the time baseof the receiver. The difference is computed based on the differencebetween the measured TBT for a transmitter and the nominal value of TBTby measuring the time of arrival of the transmissions at the receiverand comparing to the scheduled time of arrival. In the advantageousembodiment, the numbers representative of the time base differences arestored in the time registers 536 separately for each remote telemetrytransmitter and are independent from the numbers representing the timeof the next transmission,i.e. the time registers are split to hold twoindependent numbers.

Advantageously, each telemetry collection unit is furnished with a radiotransmitter that is constructed in a very similar way to that describedin conjunction with the remote telemetry transmitter operation. For aperson skilled in the art is should be apparent what alterations may berequired.

Accordingly, each transmitter of each telemetry collection unittransmits periodic messages in similar way as the remote telemetrytransmitters do with the exception that the transmission period may bemuch shorter because the telemetry collection units are not batteryoperated except during power outages. In a similar way, each telemetrycollection unit transmitter has an identification number as the remotetelemetry transmitters do. The receivers of other telemetry collectionunits can receive these messages and synchronize and track the timingand frequency of all telemetry collection units within communicationsrange. Consequently, the telemetry collection unit receiver receivesmessages from the transmitters of other telemetry collection unit in thesame way as from the sensor-transmitters. However, the telemetrycollection units are responsive in quite a different way to the messagesreceived from the telemetry collection units' transmitters as describedpreviously. It should now be apparent, that such arrangement does notprivilege any telemetry collection unit. I.e. there is no telemetrycollection unit that provides reference timing for all other telemetrycollection units. As a result, the network is robust and performs itsfunctions when one or more telemetry collection unit fails to operatecorrectly. At the same time, the communications between the telemetrycollection units benefits from the same advantages of the frequency andtime variations, i.e., immunity from interference and signal fading dueto multipath propagation.

In an advantageous embodiment, the telemetry collection units assisteach other in synchronization after one of them is powered down andpowered up again or otherwise reset. The recently reset telemetrycollection unit must have the current time and frequency information inorder to track its own and other remote telemetry transmitters. Sincethe remote telemetry transmitters may transmit very sporadically, theacquisition time may be a considerable problem. To mitigate thisproblem, other telemetry collection units that are tracking some or allthe transmitters that the recently recent unit needs to track, providethe recently reset unit with the needed time and frequency data relativeto their own transmitter timing. Since the telemetry collection unitscan transmit more often and can change the rate of the transmissions asneeded during the acquisition, this results in a considerableimprovement in the acquisition speed. For example, the recently restunit, in order to speed the synchronization with other telemetrycollection units, starts with transmitting at a rapid pace and withlonger transmission duration, thus improving the probability that atleast one telemetry collection unit receiver that continuously searchesall available frequencies, receives one such transmission.

In further refinement of the transmission method, the frequencies andthe time intervals between transmission are varied according tosequences that are individual for each collection unit in a mannerdescribed in conjunction with remote telemetry transmitters. Similarly,the time patterns are determined individually for each transmitter basedon its identification number in the same way as the frequency index, butusing different bit of the transmitter identification. The producedorthogonal frequency-time hopping patters allow many remote telemetrytransmitters and many telemetry collection units coexist with minimalmutual interference.

In some applications, an alarm packet can be transmitted at the nextstatus transmission. However, in some applications, it is desirable thatthe intervals between status transmissions are as long as possible inbattery operated transmitters. In some applications the time intervalsmay be as much as 100 seconds, thus at the worst case there would be adelay in the transmission of the alarm status that is a 100 seconds insuch case. In some applications, such as burglary alarm systems, such adelay is not acceptable. In such a case, the transmitter might transmitrepeatedly many packets in hope that the receiver that continuouslyscans all the frequencies eventually intercepts at least one suchtransmission. However, if many frequencies are used, the probability ofinterception may be small, thus requiring excessive number oftransmissions that drains the transmitter battery.

Therefore, another method is presented to remedy this in accordance withan exemplary implementation of the present invention. Accordingly, thetransmissions of alarm packets are performed at “transmissionopportunities” in accordance with a method described later in details.

Referring to FIG. 7, in accordance with advantageous implementation ofthe present invention, each transmitter establishes “transmissionopportunities” between each two consecutive status transmissions. When atransmitter needs to transmit data before the next status transmission,it transmits at the next transmission opportunity. In turn, eachreceiver that tracks this transmitter, examines the transmissionopportunities for pending transmissions. For example, while the statustransmission are transmitted at 100 second intervals, the opportunitiesmight be created at 250 ms intervals. This way the transmitter needs towait no more than 250 ms before it can transmit a packet.Advantageously, the transmission opportunities are also determined basedon time-frequency hopping sequence. This way, they experience thecommunications advantages of the frequency hopping technique.

For the purpose of this specification, the term “transmissionopportunity” is defined as: (i) time, or (ii) frequency, or (iii) code,or (iv) any combination of (i), (ii), and (iii) at which a transmissionmight occur.

Accordingly, in FIG. 7, the transmitter Txi, besides transmitting statustransmission 702 at frequency index 7 and 710 at frequency index 3, alsoestablishes transmission opportunities 701, 703, 704, 705, 706, 707,708, and 709 at frequency indexes 6, 3, 5, 2, 1, 4, 6, and 7accordingly. Similarly, transmitter Txj transmits status transmission714 and in addition establishes transmission opportunities 711, 712,714, and 715 at frequency indexes 0, 5, 3, and 2 accordingly. Thereceiver Rx, hops to catch the status transmissions at time windows 722and 733 from transmitter Txi and 728 from transmitter Txj. In addition,the receiver examines each transmission opportunity from eachtransmitter: in time windows 720, 724, 725, 727, 729, 730, and 732 fromtransmitter Txi, and in time windows 721, 723, 726 and 731 fromtransmitter Txj.

It should be pointed out that at the time window 731 there exist twoopportunities: one established by transmitter Txi at frequency 6, theother established by transmitter Txj at frequency 2. The receiver mustarbitrate which opportunity to examine, for example, based on the pastexamined opportunities or probability of traffic from each transmitter.Alternatively, the receiver might be equipped with two or more circuitsto receive at more than one frequency at the same time.

In accordance with another aspect of an advantageous implementation, thetime-frequency patterns of the transmission opportunities and the statustransmissions are related. Again, referring to FIG. 7, the frequencysequence used by the transmitter Txi is { . . . 6, 7, 3, 5, 2, 1, 4, 6,7, . . . }. The status transmission occurs at frequency 7, after whichthe next frequency is used for the next transmission opportunity. Thisway, the basic sequence is used for both the transmission opportunitiesand the status transmissions. The status transmission frequencies areestablished simply by decimating the basic sequence. In this regard, astatus transmission appears at one of the transmission opportunities.

In general, if the basic sequence length is N, and the statustransmissions are performed every M*N+1 transmission opportunities, thenthe status transmissions will occur at the same frequency pattern asthat established by the basic sequence.

In a similar way, the time hopping is introduced into the transmissionopportunities. The time hoping advantage is evidenced by the precedingexample of temporal coincidence of two opportunities. The coincidencewas not present at the next opportunity due to the time hopping.

Although there might be developed several ways of establishing thetime-frequency hopping along with ways for relating the transmissionopportunities and status transmissions, the decimation method isadvantageous for its elegance and ease of implementation.

In the illustrative embodiment described here, references are made toseveral elements such as generators, logic, registers, controloperations, etc. It is to be understood that various elements describedhere can be realized in several different forms including software andhardware in their various forms and combinations. E.g. the “logic” canbe a hardware such as a gate or memory element, or it can be a piece ofsoftware to perform a certain task. In the later case, logic simplymeans “intelligence”.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes and modifications can be effectedtherein by one skilled in art without departing from the scope andspirit of the invention as defined by the appended claims.

What is claimed is:
 1. An apparatus comprising: (1) a first telemetrycollection unit comprising: (i) a first receiver for receiving telemetryfrom each of a first plurality of remote telemetry transmitters that isassociated with said first telemetry collection unit and for receivingmessages from a second telemetry collection unit, wherein said firstreceiver comprises logic for holding data indicative of an expected timeand an expected frequency of at least one future transmission from eachof said first plurality of remote telemetry transmitters, and (ii) afirst transmitter, and (2) a second telemetry collection unitcomprising: (i) a second receiver for receiving telemetry from each of asecond plurality of remote telemetry transmitters that is associatedwith said second telemetry collection unit and for receiving messagesfrom said first telemetry collection unit, wherein said second receivercomprises logic for holding data indicative of an expected time and anexpected frequency of at least one future transmission from each of saidsecond plurality of remote telemetry transmitters, and (ii) a secondtransmitter; wherein each of said remote telemetry transmitters is fortransmissions at varied frequencies independently of any other of saidremote telemetry transmitters and independently of any of said telemetrycollection units.
 2. The apparatus of claim 1 wherein: said first andsaid second transmitter transmit at varied frequencies; and said firstreceiver includes logic for holding data indicative of an expected timeand an expected frequency of at least one future transmission from saidsecond telemetry collection unit, and said second receiver includeslogic for holding data indicative of an expected time and an expectedfrequency of at least one future transmission from said first telemetrycollection unit.
 3. The apparatus of claim 1 wherein: said firsttransmitter is for transmitting, to said second telemetry collectionunit, an identification code that is associated with at least one ofsaid first plurality of remote telemetry transmitters, and said secondtransmitter is for transmitting, to said first telemetry collectionunit, an identification code that is associated with at least one ofsaid second plurality of remote telemetry transmitters, and said firstreceiver comprises logic for holding data indicative of an expected timeand an expected frequency of at least one future transmission from saidat least one of said second plurality of remote telemetry transmitters,and said second receiver comprises logic for holding data indicative ofan expected time and an expected frequency of at least one futuretransmission from said at least one of said first plurality of remotetelemetry transmitters.
 4. The apparatus of claim 1 wherein: said firsttransmitter is for transmitting, to said second telemetry collectionunit, at least part of data indicative of an expected time and anexpected frequency that is held in said first receiver, and said secondtransmitter is for transmitting, to said first telemetry collectionunit, at least part of data indicative of an expected time and frequencythat is held in said second receiver.
 5. The apparatus of claim 1further comprising: at least one of: (a) a first network interface unitoperatively coupled to said first telemetry collection unit, and (b) asecond network interface unit operatively coupled to said secondtelemetry collection unit.
 6. The apparatus of claim 1 furthercomprising: (3) a third telemetry collection unit comprising: (i) athird receiver for receiving telemetry from each of a third plurality ofremote telemetry transmitters that is associated with said thirdtelemetry collection unit, wherein said third receiver comprises logicfor holding data indicative of an expected time and an expectedfrequency of at least one future transmission from each of said thirdplurality of remote telemetry transmitters, and wherein each of saidthird plurality of remote telemetry transmitters is for transmissions atvaried frequencies independently of any other of said remote telemetrytransmitters and independently of any of said telemetry collectionunits, (ii) a third transmitter for transmitting an alarm to said secondtelemetry collection unit, and wherein said second telemetry collectionunit, when operative, forwards said alarm to said first telemetrycollection unit.
 7. The apparatus of claim 6 wherein: said secondreceiver comprises logic for holding data indicative of an expected timeand an expected frequency of at least one future transmission from atleast one of said third plurality of remote telemetry transmitters, andsaid second receiver, when operative, receives an alarm from said atleast one of said third plurality of remote telemetry transmitters, andsaid second telemetry collection unit, when operative, forwards saidalarm to said first telemetry collection unit.
 8. A method comprising:holding, at a first telemetry collection unit, data indicative of anexpected time and an expected frequency of at least one futuretransmission from each of a first plurality of remote telemetrytransmitters that is associated said first telemetry collection unit,and receiving, at said first telemetry collection unit, telemetry fromeach of said first plurality of remote telemetry transmitters, andreceiving, at said first telemetry collection unit, messages from asecond telemetry collection unit; and holding, at said second telemetrycollection unit, data indicative of an expected time and an expectedfrequency of at least one future transmission from each of a secondplurality of remote telemetry transmitters, and receiving, at said firsttelemetry collection unit, telemetry from each of said second pluralityof remote telemetry transmitters, and receiving, at said secondtelemetry collection unit, messages from said first telemetry collectionunit, and transmitting, at each of said remote telemetry transmitters,at varied frequencies independently of any other of said remotetelemetry transmitters and independently of any of said telemetrycollection units.
 9. The method of claim 8 further comprising:transmitting at varied frequencies from said first telemetry collectionunit, and holding, at said second telemetry collection unit, dataindicative of an expected time and an expected frequency of at least onefuture transmission from said first telemetry collection unit, andtransmitting at varied frequencies from said second telemetry collectionunit, and holding, at said first telemetry collection unit, dataindicative of an expected time and an expected frequency of at least onefuture transmission from said second telemetry collection unit.
 10. Themethod of claim 8 further comprising: transmitting, from said firsttelemetry collection unit to said second telemetry collection unit, anidentification code that is associated with at least one of said firstplurality of remote telemetry transmitters, and holding, at said secondtelemetry collection unit, data indicative of an expected time andfrequency of at least one future transmission from said at least one ofsaid first plurality of remote telemetry transmitters, and transmitting,from said second telemetry collection unit to said first telemetrycollection unit, an identification code that is associated with at leastone of said second plurality of remote telemetry transmitters; andholding, at said first telemetry collection unit, data indicative of anexpected time and frequency of at least one future transmission fromsaid at least one of said second plurality of remote telemetrytransmitters.
 11. The method of claim 8 further comprising:transmitting, by said first telemetry collection unit to said secondtelemetry collection unit, at least part of data indicative of anexpected time and an expected frequency that is held in said firsttelemetry collection unit, and transmitting, by said second telemetrycollection unit to said first telemetry collection unit, at least partof data indicative of an expected time and an expected frequency that isheld in said second telemetry collection unit.
 12. The method of claim 8further comprising at least one of: (a) transmitting via a first networkinterface unit from said first telemetry collection unit, and (b)transmitting via a second network interface unit from said secondtelemetry collection unit.
 13. The method of claim 8 further comprising:holding, at a third telemetry collection unit, data indicative of anexpected time and an expected frequency of at least one futuretransmission from each of a third plurality of remote telemetrytransmitters that is associated said third telemetry collection unit,and receiving at said third telemetry collection unit telemetry fromeach of said third plurality of remote telemetry transmitters; andtransmitting an alarm from said third telemetry collection unit to saidsecond telemetry collection unit; and transmitting said alarm form saidsecond telemetry collection unit to first said telemetry collectionunit.
 14. The method of claim 8 further comprising: holding at saidsecond telemetry collection unit, data indicative of an expected timeand frequency of at least one future transmission from at least one of athird plurality of remote telemetry transmitters that is associated witha third telemetry collection unit, and receiving an alarm at said secondtelemetry unit from said at least one of said third plurality of remotetelemetry transmitters, and forwarding said alarm from said secondtelemetry collection unit to said first telemetry collection unit.
 15. Atelemetry collection unit comprising: (i) a receiver for receivingtelemetry from a first plurality of remote telemetry transmitters thatis associated with said telemetry collection unit and for receiving analarm from a second telemetry collection unit, wherein (a) each of saidremote telemetry transmitters is for transmissions at varied frequenciesat its own pace independently of any other of said remote telemetrytransmitters and independently of any of said telemetry collectionunits, and wherein (b) said receiver comprises logic for holding dataindicative of an expected time and an expected frequency of at least onefuture transmission from each of said first plurality of remotetelemetry transmitters; and (ii) a transmitter for forwarding said alarmto at least one third telemetry collection unit.
 16. The telemetrycollection unit of claim 15 wherein said transmitter transmits at variedfrequencies, and said receiver includes logic for holding dataindicative of an expected time and an expected frequency of at least onefuture transmission from at least one other telemetry collection unit.17. The telemetry collection unit of claim 15 wherein said receiver isfor receiving a second alarm from at least one of a second plurality ofremote telemetry transmitters that is associated with said secondtelemetry collection unit; and said transmitter is for forwarding saidsecond alarm to at least one third telemetry collection unit; whereinsaid receiver further comprises logic for holding data indicative of anexpected time and an expected frequency of at least one futuretransmission from said at least one of said second plurality of remotetelemetry transmitters, and each of said second plurality of remotetelemetry transmitters is for transmissions at varied frequencies at itsown pace independently of any other of said remote telemetrytransmitters and independently of any of said telemetry collectionunits.
 18. The telemetry collection unit of claim 15 wherein saidreceiver is for receiving, from a second telemetry collection unit, anidentification code of at least one of a second plurality of remotetelemetry transmitter that is associated with said second telemetrycollection unit; and said transmitter is for transmitting anidentification code of at least one of said first plurality remotetelemetry transmitters to at least one other telemetry collection unit.19. The telemetry collection unit of claim 15 wherein an alarmtransmitted by said telemetry collection unit comprises anidentification code for distinguishing alarms originated from differentsources and at different instances.
 20. The telemetry collection unit ofclaim 15 wherein said transmitter is for transmitting at least part ofdata indicative of an expected time and an expected frequency that isheld by said receiver to at least one other telemetry collection unit.21. The telemetry collection unit of claim 15 wherein: said telemetrycollection unit is operatively coupled to a network interface unit toforward said alarm to a central monitoring facility.
 22. A method ofoperating a telemetry collection unit comprising: holding dataindicative of an expected time and an expected frequency of at least onefuture transmission from each of a first plurality of remote telemetrytransmitters that is associated with said telemetry collection unit,wherein each of said first plurality of remote telemetry transmitters isfor transmissions at varied frequencies at its own pace independently ofany other of said remote telemetry transmitters and independently ofsaid telemetry collection unit, and receiving telemetry from each ofsaid first plurality of remote telemetry transmitters in accordance withsaid data, and receiving an alarm from a second telemetry collectionunit; and forwarding said alarm to at least one third telemetrycollection unit.
 23. The method of claim 22 further comprising:transmitting from said telemetry collection unit at varied frequencies,and holding, at said telemetry collection unit, data indicative of anexpected time and frequency of at least one future transmission from atleast one other telemetry collection unit.
 24. The method of claim 22further comprising: holding data indicative of an expected time and anexpected frequency of at least one future transmission from at least oneof a second plurality of remote telemetry transmitters that isassociated with said second telemetry collection unit, and receiving asecond alarm from said at least one of said second plurality of remotetelemetry transmitters; and forwarding said second alarm to at least onethird telemetry collection unit; wherein each of said second pluralityof remote telemetry transmitters is for transmissions at variedfrequencies at its own pace independently of any other of said remotetelemetry transmitters and independently of any of said telemetrycollection units.
 25. The method of claim 22 further comprising:receiving, from said second telemetry collection unit, an identificationcode of at least one of a second plurality of remote telemetrytransmitter that is associated with said second telemetry collectionunit; and transmitting an identification code of at least one of saidfirst plurality remote telemetry transmitters to at least one othertelemetry collection unit.
 26. The method of claim 22 furthercomprising: including in an alarm transmitted by said telemetrycollection unit an identification code for distinguishing alarmsoriginated from different sources and at different instances.
 27. Themethod of claim 22 further comprising: transmitting, by said telemetrycollection unit, at least part of data indicative of an expected timeand frequency that is held by said telemetry collection unit to at leastone other telemetry collection unit.
 28. The method of claim 22 furthercomprising: forwarding said alarm to a central monitoring facility usinga network interface unit.